Mast cells (MCs) are important in allergic diseases, tissue injury, and protection from infections. MC numbers and their effector functions are dynamically regulated by constituents of the tissue microenvironment. This application for continued support focuses on the role of cysteinyl leukotrienes (cys-LTs) and their receptors in the regulation of MC function. Human MCs (hMCs) expressed both of the known receptors for cys-LTs (CysLTI and CysLT2 receptors). IL-4 regulates cys-LT-dependent signaling events as well as CysLT2 receptor expression, and CysLTI and CysLT2 receptors exhibit differential and complementary functions as defined by studies of null strains of mice. We have recently uncovered an unanticipated function for cys-LTs; namely, as key regulators of an autocrine/intracrine signaling loop that is essential for the expansion of MC numbers (reactive mastocytosis) occurring in the lung and intestinal mucosal surfaces in Th2-biased immune responses in vivo. Both leukotriene C4 synthase-null mice (which cannot generate cys-LTs) and CysLTI receptor-null mice virtually lack MCs in the inflamed epithelial surfaces of the bronchi and gut. Moreover, we have demonstrated that cys-LTs are necessary as autocrine growth factors for proliferative responses of human and mouse MCs to IL-4. A nucleotide, UDP, mimics the effects of cys-LTs as co-mitogenic growth factors for hMCs in vitro, acting by a pathway that requires CysLTI receptors to induce activation of the ERK MAP kinase cascade. We have now demonstrated that CysLTI receptors on MCs constitutively form heterodimers with CysLT2 receptors and with UDP-selective P2Y6 receptors, respectively, and that each heterodimer differentially regulates the function of the therapeutically relevant CysLTI receptor. Finally, we have successfully derived mice lacking P2Y6 receptors. The Specific Aims are: 1. To determine the molecular basis and functional consequences of dimerization of P2Y6 and CysLT2 receptors with the CysLTI receptor, and; 2. To define the mechanisms through which P2Y6/CysLT1 receptors amplify stem cell factor (SCF)-dependent growth of hMCs, and mediate reactive mastocytosis in vivo using newly developed null mouse strains. This proposal seeks to understand how a class of chemicals called cysteinyl leukotrienes control the function of mast cells, a type of immune cell. Since both mast cells and cysteinyl leukotrienes are involved in asthma and are also believed to be important in heart disease and other conditions, these studies are anticipated to reveal potential new treatments for these common diseases.